Increased growth rates are typically associated with higher economic returns to beef producers. Consequently, methods to improve growth rate in cattle are of significant benefit to producers. Prior art methods of increasing growth has included such approaches as the use of hormone implants (Bagley et al., 1989), sub-therapeutic levels of antibiotics (Schumann et al., 1990) and by selective breeding based on expected progeny differences (EPD) (Kress et al., 1977). However, hormone implants and the use of antibiotics are becoming unpopular and may be banned in North America in the near future. Therefore, alternative methods of improving growth rates of cattle that don't require artificial forms of stimulation will become increasingly important and desirable in the industry.
Corticotropin releasing hormone (CRH) indirectly causes the release of glucocorticoids (Dunn and Berridge, 1990), naturally occurring hormones that are suggested to be growth inhibitors (Sharpe et al., 1986). Although commonly referred to as a “stress-related hormone”, CRH is released from the hypothalamus, an area of the brain known to be involved in appetite control. The release of CRH regulates appetite via two distinct mechanisms: (1) indirectly triggering the release of pro-opiomelanocortin (POMC), and (2) by increasing the production of leptin (FIG. 1)
The up-regulation of POMC levels leads to increased synthesis of alpha melanocyte stimulating hormone (αMSH) which, when bound to the melanocortin-4 receptor, reduces appetite (Marsh et al., 1999). The increase of leptin, which is induced by glucocorticoids,  reduces appetite by four other interactions (FIG. 1). Primarily leptin acts to decrease the levels of neuropeptide Y, an appetite stimulant. Leptin also acts to increase POMC levels, an agonist for the melanocortin 4-receptor (MC4R); decrease the levels of antagonist agouti related protein (AGRP); and increase the production of CRH (Houseknecht et al., 1998; Marsh et al., 1999; Pritchard et al., 2002).
The CRH gene comprises two exons, however only exon 2 is translated and codes for the pre-pro-protein (Roche et al., 1988; Shibihara et al., 1983). The CRH gene has been mapped to chromosome 14 (Barendse et al., 1997), and the results of quantitative trait linkage (QTL) mapping suggested an association between a locus for post-natal growth identified on chromosome 14 and the CRH gene (Buchanan et al., 2000). In addition, we previously reported a non-conserved amino acid substitution at position 77 (CRH77) in the pro-peptide region of CRM and showed an association with post-natal growth in beef cattle (Buchanan et al., 2002b).
The POMC pro-hormone peptide is an integral component of the appetite regulation pathway (FIG. 1) and has also been identified by QTL analysis in our unpublished studies as a positional candidate gene for average daily gain and carcass weight. We identified a single nucleotide polymorphism (SNP) in the POMC gene that is translationally silent and used the SNP to map the POMC gene to chromosome 11 in beef cattle (Thue et al., 2003), confirming its position to previously identified QTL loci. We also identified SNPs in two other genes integral to this pathway, leptin and MC4R (Buchanan et al., 2002a; Thue et al., 2001).
We have recently identified a novel SNP in the CRH gene, at position 4 of the signal sequence, equivalent to position 22 of the sequence defined in SEQ ID NO: 1. Together with the existing gene tests for POMC, MC4R and LEP (Buchanan et al., 2002a; Buchanan et al., 2002b; Thue et al., 2001; Thue et al., 2003) we genotyped a group of 256 steers. Our results show that knowledge of genotypes of cattle, with respect to these particular genes, can be used to better predict growth and yield during beef production. 